IIF you hold the desire to die to be rational. I can believe that 2 + 2 = 5, but it doesn't. The desire to die may be derived not from an actual desire, but an expression of mental or physical illness which could be treated in such a way that I (rationally) do not desire to die (right now).

Whether there are valid exceptions or mere failures of treatment is what most people are inclined to debate (plus the difficulty in discerning rational consent, free of pressure or other external failures).